In case you haven’t been paying attention, my about page indicates that I am about to graduate with a degree in Physics, specifically the experimental kind. I still have an unalloyed love of physics and the natural sciences, and so from time to time you’ll find that posts on this blog turn to the natural sciences. This is the first of these.
This post was brought on, in part, because of a game of Trivial Pursuit I played with my friends from University on holiday in the Lake District (which, incidentally, is why I haven’t posted recently). Aside from reminding my friend and I of why we hate Trivial Pursuit, a small incident in the game stayed with me well past the game’s conclusion. The incident occurred as follows:
One team was asked the question: “What colour is the light from the hottest stars?” (For the benefit of those of you who do not remember your high school physics classes, the answer to this question is blue. The physics of this I will touch upon later in this post.) This question is perfectly fair and legitimate. The team so-asked were unfamiliar with the notion that the stars had different colours, and ended up providing as a guess the answer “Indigo.” The person who asked the question responded with the hint: “Can you give me a primary colour?”
At the time, I was displeased with the providing of such a hint as a response to an incorrect answer. Afterwards, however, what stuck with me was not the blatant cheating (OK, so it stuck with me a bit), but the nature of the hint: “Can you give me a primary colour?”
Get Over It
“What’s wrong with that as a hint?”, I hear you ask. “Blue is a primary colour, so the hint was leading towards the correct answer!” Considered in the context of the game itself, the hint is totally understandable, if outside the rules a little bit.
Here’s what bothered me: stars don’t only come in primary colours. The fact that blue, a possible (and commonish) stellar colour, is a primary colour is pure happenstance. The colours stars can have fall on and in-between the following colours: blue, white, yellow, orange, red.
“Alright, so what? So the hint had no relationship to the scientific reality of the situation. Why does that matter?”, you say. Here’s why.
Modern scientific understanding is built on the work done by nearly innumerable scientists past. For each tiny bit of understanding added to the vast library of scientific knowledge of the natural world, for every scientist who is credited with the explanation or discovery of a given phenomenon, hundreds of scientists laboured in obscurity in an attempt to explain it, and hundreds more provided the earlier knowledge required to build the foundational knowledge for the explanation.
Huge praise is heaped, quite rightly, on scientists who discover and explain the world. Einstein, Hawking, and Newton are names well-known to the general public. To those types, we can add those whose fame lies largely within the field: Feynman, Bohr, and Bragg (Bragg Junior and Bragg Senior), for example. These men and women were and are great minds, giants amongst our species, who provided invaluable assistance to humanity’s ongoing quest to understand and explain the world around us, and the praise heaped upon them at least as deserved as that provided to any other person in the history of humanity.
However, behind each of these giants stand the legion of the unsung; those scientists who laboured for a lifetime to attempt to push the boundaries of human knowledge just a bit further. Their names aren’t referred to in discussions of notable scientists or in popular science books, and their names are never the answer to questions in Trivial Pursuit. They devoted their lives to the pursuit of a worthy goal, and earned themselves exactly no fame.
As a result, when a phenomenon that required years and buckets of ingenuity to explain is belittled or reduced to less than it is, those who do that reduction are injuring not just those whose names are attached to the phenomenon in question but those who lived and died anonymously in search of the explanation.
It is important to remember that off-hand devaluations of science are more than just insults to reality: they are insults to hundreds of dead unknowns. And that’s just poor taste.
The Physics
After having been on that rant, I feel like I should step aside for a moment to indicate that the physics of the colour of stars is both interesting and poetic. I won’t go into it in too much depth here, in part because I’m not as familiar with the subject as I’d like to be, but I’ll briefly stop to walk through some of the core concepts.
We can begin by stating someone that many of you will already be aware of or have noticed: when you heat things up, if they get hot enough they glow. This is easily demonstrated: for example, place a metal object into a fire for a period of time. Left long enough, it will begin to glow red.
If you experiment with enough care, you’ll begin to find that the colour something glows depends on how hot you get it. As you heat that metal object further it’ll begin to get whiter, passing through orange and yellow along the way.
This pair of observations would lead a sufficiently adventurous mind to suggest that there is probably a relationship between how hot something is and the colour it glows. If you add to this our knowledge of the electromagnetic spectrum, you might be able to extend your hypothesis a little further. For instance, you could say that a hotter object appears to be a colour of shorter wavelength than a cooler object. If you were really bold, that would lead you to conclude that even when objects don’t appear to be glowing, they are still ‘glowing’ in the infrared, which manifests to our senses as heat.
The next few steps are less obvious, and I shall have to state them rather than give you examples. First, we invoke the theoretical construct of the ‘perfect blackbody’. This is an idealised (read: cannot exist) object that absorbs perfectly. Because physics is full of lots of exciting symmetries, a blackbody also radiates perfectly. Stars, as it turns out, are excellent approximations of perfect blackbodies, and so you can model the way they emit light as being the same as a perfect blackbody (almost).
Max Planck, a very renowned physicist, proposed a law (specifically an equation) that describes the way perfect blackbodies radiate at a given temperature. This law is called Planck’s Law. For the purposes of this discussion, it can be summarised in the following points:
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For an object of a given temperature, there is a maximum possible frequency of light that can be emitted.
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For an object of a given temperature, there is a frequency of light that is emitted more than any other.
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Hotter objects radiate more light in total than cooler objects.
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Frequencies of light below the maximum are always emitted in some quantity, depending on how hot the object is.
From these points we can explain 95% of the colours of stars. Cool stars do not emit any blue light at all, because blue light has a higher frequency than red light, and they’re too cold. Cool stars, therefore, appear red. The hotter they are, the more of the higher frequencies they emit, and the bluer they get.
This series of observations leads to the relationship between star colour and star temperature, and incidentally explains the way things behave on earth too.
You said 95%?
I did. So, here’s a thing. If you go look at the Planck’s Law page again, you’ll notice that what I said in point 2 above applies right through the spectrum. As a result, there is a temperature of star that radiates mostly green light: around 6000 Kelvin, in fact. The Sun’s surface temperature for the purpose of blackbody calculations is about 5780K, which means the Sun radiates mostly green light. Why, then, don’t we see the Sun as green, but instead as white? Furthermore, given how average our Sun is, why don’t we see any green stars at all?
To answer this question, we have to go interdisciplinary, and turn to our friends, the Biologists. Specifically, we have to consider the theory of evolution.
We evolved on this planet, with our Sun as the sole source of light. In particular, our eyes evolved in an environment where the light emitted by our Sun was the sole source of light available to them. Given that the Sun emits more green light than any other colour, it is evolutionarily advantageous to become more sensitive to green light than other colours, as there will always be more of it to detect.
Put more simply, evolution means that our eyes have what physicists and mathematicians would call a non-linear response to luminous intensity: we are more sensitive to some colours than others. Thus, what we perceive as ‘white light’ is not an equal mixture of all wavelengths of light, but contains more of some than others.
If we had eyes that were equally sensitive at all wavelengths, we would end up seeing stars of all colours, green included. This bit of information provides the final clue to the colours of stars.